Conventionally, the HIP method, which is a pressing method using a hot isostatic pressing device, has been known. In this HIP method, the following processing is performed: a workpiece such as a sintered product (ceramics and the like), a casting, or the like is heated to a temperature higher than a recrystallization temperature of the workpiece in an atmosphere of a pressure-medium gas that is set at a high pressure of several tens to several hundreds of MPa. Therefore, the HIP method is characterized in that residual pores in the workpiece can be extinguished. The HIP method, therefore, has been recognized to have effects such as the improvement of mechanical properties, the reduction of variations of properties, and the improvement of the yield, and has become used widely in industries in these days.
Incidentally, in real production sites, speedup of processing has been desired earnestly, and it is considered that, for this purpose, it is necessary and indispensable to carry out the cooling step in a short time, which is a time-consuming step among the steps for HIP processing. To cope with this, for the conventional hot isostatic pressing device (hereinafter referred to as a “HIP device”), various methods for improving the cooling speed while holding the inside of a furnace in a thermally uniform condition have been proposed.
For example, Patent Document 1 discloses a HIP device that includes the following: a high-pressure container that houses a workpiece; a gas-impermeable inner casing that is provided so as to surround the workpiece in the inside of the high-pressure container; a gas-impermeable outer casing that is provided outside the inner casing so as to surround the same; and a heating means that is provided inside the inner casing so as to form a hot zone around the workpiece. The hot zone is formed inside the inner casing, and is held in a heat insulated manner by the inner casing and the outer casing. The isostatic pressing processing is performed by using a pressure-medium gas stored in the hot zone.
This HIP device further includes a cooling means that cools the workpiece in the hot zone by circulating the pressure-medium gas in the inside of the high-pressure container. This cooling means includes a first cooling means and a second cooling means.
The first cooling means performs the cooling by circulating the pressure-medium gas in such a manner that the pressure-medium gas forms a first circulating flow. In this first circulating flow, the pressure-medium gas is cooled while being led so as to flow between the inner casing and the outer casing from below to above, guided from an upper part of the outer casing toward the outside of the outer casing, and further guided from above to below along an inner circumference surface of the high-pressure container. The pressure-medium gas thus cooled is returned from below the outer casing to between the inner casing and the outer casing.
The second cooling means cools the pressure-medium gas by circulating the pressure-medium gas in such a manner that the pressure-medium gas forms a second circulating flow. In the second circulating flow, the pressure-medium gas in the hot zone is guided to the outside of the hot zone so as to join the pressure-medium gas that is forcibly circulated by the above-described first cooling means, whereby the pressure-medium gas is cooled. In this way, the circulation of the pressure-medium gas is performed so that a part of the pressure-medium gas thus cooled is returned to the hot zone.
In the above-described hot isostatic pressing device of Patent Document 1, a part of the pressure-medium gas composing the first circulating flow is caused to join the second circulating flow from a lower part of the hot zone with use of a fan and an ejector, and the pressure-medium gas thus joining cools the hot zone while circulating in the hot zone. This makes it possible to eliminate a temperature difference between the upper and lower parts of the furnace that occurs in the cooling process, thereby effectively cooling the inside of the furnace.
Further, Patent Document 2 discloses a hot isostatic pressing device that carries out a cooling step in a short time by taking out a pressure-medium gas in a high-pressure container to outside the container, cooling the same outside the container, and thereafter returning the same into the container.
In the case of the HIP device disclosed in the Patent Document 1, the high-temperature gas in the hot zone is guided toward above outside the heat-insulation layer, the high-temperature gas and an inner surface of the container are caused to exchange heat while this high-temperature gas goes down through the clearance between the container and the heat-insulation layer whereby the temperature of the high-temperature gas falls, and the gas having a lower temperature as a result is circulated in the hot zone, all of which makes it possible to quickly cooling the hot zone. In particular, it can be considered that in the HIP device disclosed in Patent Document 1, the pressure-medium gas forming the first circulating flow is sufficiently cooled to such a low temperature that the soundness of the pressure container and the like can be maintained.
This prior art, however, involves a problem that the temperature of electrical components and the like provided in a lower part of the high-pressure container cannot be dropped enough. To describe more specifically, in the lower part of the high-pressure container, there are a fan and a motor having a rotation control function for promoting the circulation of gas, a valve for gas flow control, and an actuator for the same, or contact points for an electric heater and a thermocouple for measuring temperature, and the like, and it cannot be considered that the temperature of the lower part of the high-pressure container is sufficiently low from the viewpoint of the heat-resisting properties of these members. Accordingly, there is a possibility that these electrical components are burnt out.
This problem tends to be more serious when, as is the case with the device disclosed in Patent Document 1, the low-temperature pressure-medium gas that forms the first circulating flow, and the high-temperature pressure-medium gas that forms the second circulating flow are caused to join first, and the pressure-medium gas after the joining is caused to go down along an inner circumference surface of the high-pressure container.
For example, in a quick cooling method that has been performed conventionally, only the pressure-medium gas that forms the first circulating flow, which goes down along the inner circumference surface of the high-pressure container without forming the second circulating flow, thereby coming to have a lower temperature, is guided into the hot zone, and the pressure-medium gas thus having a higher temperature is guided again to the inner circumference surface of the high-pressure container. In such a quick cooling method, the circulation flow rate itself is small. Therefore, the temperature of the circulation flow after going down along the inner circumference surface of the high-pressure container is low, and is already cooled to such a temperature that electrical components should not be burnt out. In the case where the pressure-medium gas that forms the first circulating flow and the pressure-medium gas that forms the second circulating flow are mixed first and the pressure-medium gas thus formed by mixing is caused to flow down along the inner circumference surface of the high-pressure container so as to be cooled, however, the flow rate of the circulation gas is large, and there is a possibility that the temperature of the pressure-medium gas does not fall sufficiently. This makes the possibility higher that the high-temperature pressure-medium gas flows to the lower part of the high-pressure container, and causes the electrical components to be burnt out.
Such a cooling method that pressure-medium gases are mixed first are cooled is used often in the case where the amount of circulation of the pressure-medium gas that forms the first circulating flow is increased so that the cooling speed is increased, as is the case with the HIP device of Patent Document 1. The HIP device of Patent Document 1, therefore, has a problem that there is a greater possibility that electrical components are burnt out depending on driving conditions.